1. Field of the Invention
This invention relates to a method and an apparatus for cutting a hard and brittle material to be worked, such as ceramics, a silicon single crystal material and a rock crystal material, with a high degree of precision and high efficiency.
2. Description of the Prior Art
Conventional methods for cutting hard and brittle materials such as ceramics, silicon single crystals, rock crystals, etc. include a method in which the material to be cut is cut by rotating a diamond wheel which is prepared with grains of diamond buried in a thin disc at a high speed and by pushing the rotating disc against the material to be cut; and a method in which either a tool of a linear shape such as wire or a thin strip-shaped cutting tool called a multi-blade is caused to make a reciprocating motion at a very low frequency not exceeding several Hz and at a stroke length of one hundred mm or thereabout through a suitable guide while, at the same time, a machining or work liquid containing abrasive grains is continuously supplied to the cutting part of the material during a cutting process. In the case of the former in which a high speed rotating diamond cutter is employed, the work efficiency is high. However, the use of the diamond wheel necessitates an after-work process such as lapping because of saw marks produced on the cut surface of the material by the diamond wheel. Besides, the diamond wheel is expensive and thus has much affected the cost of the product thus obtained. In the latter method where the cutting work is to be accomplished by causing a thin cutting tool to make a reciprocating motion with the supply of a grain suspension, the work efficiency is very low though the precision of the cut surface of the material might be satisfactory. Besides, with the latter method employed, uneven distribution of the cutting powder grains within the suspension has often resulted in a poor finishing surface.
Meanwhile, there has been developed a method as disclosed in Japanese patent application Laid-Open No. 51-123987 in which thin wire is wound round a plurality of multi-groove pulleys; the wire is caused to make a reciprocating motion by a drive mechanism disposed outside; further, a vibration is transmitted to the wire through the pulleys; and during a cutting process, a hard and brittle material is cut into thin plates or sliced with a mixture consisting of grains of cutting powder and oil supplied to the cutting part of the material to be cut. In this method, however, inadequate arrangement of the cutting pulleys tends to result in breakage of the wire which is employed as cutting tool. This lowers the work efficiency. Further, since an excessively large tension cannot be applied to the wire, it is inevitable that the wire is curved during the cutting work on the material. This results in fine streaky scars on the cut surface or the material. Besides, where the wire is a tungsten wire, it is so twisted when it is rewound that reuse of it is hardly possible. This causes a great increase in cost.
In an attempt to obviate these shortcomings of the methods of the prior art, the present inventors conducted studies and eventually came up with a new method of cutting a hard and brittle material. This method was characterized in that a low frequency vibration in the direction of the cutting edge of a strip-shaped flat cutting tool and a reciprocating motion of a given stroke length in the same direction are simultaneously imparted to the cutting tool in a superimposing manner. This method has been disclosed in Japanese patent application Laid-Open No. 52-81190. Through further studies, however, the new method has been presenting the following problem when it is to be embodied in a practical cutting apparatus: A reciprocating motion and a small-amplitude vibration are imparted to a strip-shaped flat cutting tool in accordance with the new method as disclosed in the previous patent application. However, the impartment of the reciprocating motion and the low frequency vibration to the strip-shaped flat cutting tool during cutting work necessitates, for actual application of the method, mounting of the cutting tool on a rigid tool frame with tension applied to the cutting tool up to a value close to an endurable limit thereof in order to have the thin flat cutting tool apparently serve as a rigid body. This necessitates provision of a stretching block for mounting the cutting tool in a stretched state. Such arrangement then results in a considerably large weight, say 50 kg f, of the tool mounting frame. Then, the impartment of a vibration up to a maximum value of 200 Hz to the tool frame is uneconomical in respect of the capacity of a vibration exciter and the rigidity of the machine. The method thus tends to be impracticable. Assuming that the weight of the tool frame W is 50 kg f; the cycle of vibration or oscillation frequency f is 60 Hz; the amplitude of the vibration a is 1 mm; and a harmonic vibration x is asin 2.pi. ft, if such a vibration is to be applied to the tool frame, a calculated gravity acceleration is as follows:
Gravity acceleration g=9800 mm/sec.sup.2
Therefore, the maximum value of an inertia force F max produced by the vibration can be obtained as shown below: ##EQU1##
Meanwhile, assuming that the weight of the material to be cut together with the material mounting mount is W'=3 kg f, the maximum value of an inertia force thereof F' max can be obtained in the following manner: ##EQU2##
In view of the above, it may be considered impossible to impart a vibration to the tool frame in terms of the rigidity of the whole apparatus and economy.
To solve this problem, the present inventors have further invented a practicable improvement over the above-stated invention for which the above-stated patent application was filed (Japanese patent application Laid-Open No. 52-81190). For this improvement another patent application has been filed under Japanese patent application No. 53-161780. In accordance with this invention, a reciprocating motion and a small-amplitude vibration are arranged to be imparted to the material to be worked on, which is of a smaller mass than a tool mounting frame, both in directions in parallel with the cutting edge of a flat cutting tool in a superimposed manner. The improvement has been found not only capable of solving the above-stated problem but also capable of improving the precision of the cut surface and the work efficiency.